In a wavelength division multiplexing optical communication system, information is carried by multiple channels, each with a distinct wavelength. It is often necessary to add or drop a wavelength channel in optical links or networks. This can be achieved by a wavelength switch that sends different wavelength channels to different locations. These switches are often termed “add-drop switches” because they extract certain channels delivered from an input port and send the extracted channels to a “drop” port whilst, simultaneously, they receive certain replacement channels from an “add” port and send these added channels to an output port. The non-dropped or through-going channels originally from the input port, herein referred to as “express” channels, are also sent to the output port. Such add-drop switches are also referred to as “add-drop multiplexers” because, they must combine or multiplex channels as part of their channel adding operation.
FIG. 4 is a schematic diagram of the architecture of a conventional optical add-drop switch 400, wherein the added channels are de-multiplexed upon entering the apparatus and wherein the removed channels are multiplexed upon exiting the apparatus. The conventional optical add-drop switch 400 comprises an input port 401, an output port 403, a first de-multiplexer (DEMUX) 402a optically coupled to the input port 401, a first multiplexer (MUX) 404a optically coupled to the output port 403, a plurality of 2×2 optical switches 406a–406c, optically coupled between the first DEMUX 402a and the first MUX 404a, an add port 405, a drop port 407, a second DEMUX 402b optically coupled between the add port 405 and to each of the 2×2 optical switches 406a–406c, and a second MUX 404b optically coupled between the drop port 407 and to each of the 2×2 optical switches 406a–406c. 
The optical couplings 413a, 413b and 413c of the conventional optical add-drop switch 400 optically couple the first DEMUX 402a to switch inputs of the respective 2×2 optical switches 406a, 406b and 406c. The optical couplings 415a, 415b and 415c optically couple switch outputs of the 2×2 optical switches 406a, 406b and 406c, respectively, to the first MUX 404a. The optical couplings 409a, 409b and 409c optically couple the second DEMUX 402b to switch inputs of the respective 2×2 optical switches 406a, 406b and 406c. The optical couplings 411a, 411b and 411c optically couple switch outputs of the 2×2 optical switches 406a, 406b and 406c, respectively, to the second MUX 404b. 
An input composite optical signal comprising a plurality of wavelength division multiplexed optical channels is input to the first DEMUX 402a from the input port 401. The first DEMUX 402a separates each of the channels to a unique respective one of the plurality of 2×2 optical switches 406a–406c via a respective one of the optical couplings 413a–413c. Further, an add-port composite optical signal comprising a plurality of wavelength division multiplexed optical channels is delivered to the second DEMUX 402b from the add port 405, wherein the wavelengths of the add-port composite optical signal are a subset of the wavelengths of the input composite optical signal. The second DEMUX 402b separates each of these added channels to a unique respective one of the plurality of 2×2 optical switches 406a–406c via a respective one of the optical couplings 409a–409c. 
Each 2×2 optical switch 406a, 406b, 406c receives a respective channel of the input composite signal and may receive a channel of the add-port composite optical signal comprising the same wavelength as the channel of the input composite optical signal. Each 2×2 optical switch may be in either one of two states—a “cross” state or a “bar” state, for which the operation of switch 406a is utilized as an example. In the cross state, the channel from optical coupling 413a is switched to optical coupling 411a whilst the added channel from optical coupling 409a is switched to optical coupling 415a. Conversely, in the bar state, the channel from optical coupling 413a is switched to optical coupling 415a and, in general, there is no added channel from the optical coupling 409a. Therefore, when a switch is in its cross state, a channel originally from the input port 401 is directed to the second MUX 404b and thence to the drop port 407 and a replacement channel from the add port is directed to the first MUX 404a and thence to the output port 403. However, when the switch is in its bar state, there is generally no channel from the add port and the channel originally from the input port 401, which is, in this instance, termed an “express” channel, is directed to the first MUX 404a and thence to the output port 407. Therefore, the conventional optical add-drop switch 400 drops and adds channel in pairs and the adding and dropping of each wavelength is independent of the adding and dropping of every other wavelength.
The conventional add-drop switch, although capable of performing its intended function, comprises two multiplexers and two de-multiplexers as well as numerous 2×2 switches. Although only three such 2×2 switches are illustrated in FIG. 4, the conventional add-drop switch will, in general, include one such switch for each one of every possible WDM channel, which may comprise sixteen or more channels. The utilization of such a plurality of 2×2 switches yields a complex and bulky apparatus and can cause a high level of insertion loss, since extra optical components or optical couplings between components induce additional losses.
Accordingly, there exists a need for an improved optical add-drop switch. The improved switch should provide a high extinction ratio and low loss. The present invention addresses such a need.